When one is born again after trusting Jesus Christ as his Saviour, there is a new nature created in the image of Christ. This is the New Man, filled with spiritual insight and strength because of the Holy Spirit who now resides in him. But the Old Man, the sin nature of each one of us is still there as it can only be removed if we are apart from the physical body we now have.
Hence daily it is a constant battle of the New Man vs the Old Man in us. The New Man has victory if we listen and do the will of God daily. But if we listen to the temptations of the evil one and obey them, we lose and the Old Man in us wins.
Thanks be to God for the victory that we have in Christ who now cancels our sin nature and replaces that with His pure nature.
SIN is, as the middle letter represents, the 'I' nature - self-centred, egoistic and proud.
CHRIST now replaces our SIN nature when we trust and submit to Him. Notice that the I is still there, as trusting Christ does not make us a robot but we are still unique in our person-hood. But now the I does not occupy the centre position but is subsumed in CHRIST as He lives in us through the Holy Spirit.